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Restructure GDT. Add sysenter support #290

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merged 6 commits into from
Aug 30, 2023
Merged

Restructure GDT. Add sysenter support #290

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eeecf75
entry: syscall_handler_entry save/restore all regs
sktt Aug 30, 2023
3817186
segment: restructure and document GDT selectors
sktt Aug 27, 2023
6796609
usermode: adjust MSR_STAR.user_cs
sktt Aug 27, 2023
e78424e
entry, usermode: add sysenter handler
sktt Aug 27, 2023
ec322c4
usermode: add sysenter and syscall_mode selection
sktt Aug 27, 2023
22cd3a1
usermode: support int 80h syscalls
sktt Aug 28, 2023
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45 changes: 42 additions & 3 deletions arch/x86/entry.S
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Expand Up @@ -180,9 +180,10 @@ ENTRY(enter_usermode)
SAVE_ALL_REGS
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PUSHF

/* Save stack pointer onto per-cpu */
/* Save user stack pointer onto per-cpu */
mov %_ASM_DX, %gs:(usermode_private)

/* Now switch stack and address space */
syscall_to_usermode

/* SS + SP */
Expand All @@ -201,7 +202,7 @@ ENTRY(enter_usermode)
END_FUNC(enter_usermode)

ENTRY(syscall_handler_entry)
SAVE_CALLEE_SAVED_REGS
SAVE_ALL_REGS
syscall_from_usermode

cmp $SYSCALL_EXIT, %_ASM_AX
Expand All @@ -218,9 +219,47 @@ ENTRY(syscall_handler_entry)
pop %_ASM_CX

syscall_to_usermode
RESTORE_CALLEE_SAVED_REGS
RESTORE_ALL_REGS
SYSRET
END_FUNC(syscall_handler_entry)

ENTRY(sysenter_handler_entry)
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SAVE_ALL_REGS
syscall_from_usermode

MASK_USER_FLAGS

cmp $SYSCALL_EXIT, %_ASM_AX
jz syscall_exit

mov %_ASM_DI, %_ASM_DX
mov %r10, %_ASM_CX
mov %_ASM_AX, %_ASM_DI
call syscall_handler

syscall_to_usermode
RESTORE_ALL_REGS

SYSEXIT
END_FUNC(sysenter_handler_entry)

ENTRY(int80_handler_entry)
SAVE_ALL_REGS
syscall_from_usermode

MASK_USER_FLAGS

cmp $SYSCALL_EXIT, %_ASM_AX
jz syscall_exit

mov %_ASM_DI, %_ASM_CX
mov %_ASM_AX, %_ASM_DI
call syscall_handler

syscall_to_usermode
RESTORE_ALL_REGS
IRET
END_FUNC(int80_handler_entry)
GLOBAL(end_usermode_helpers)

SECTION(.text.user, "ax", 16)
Expand Down
4 changes: 2 additions & 2 deletions arch/x86/real_mode.S
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Expand Up @@ -164,15 +164,15 @@ END_FUNC(_prot_to_real)
mov rmode_stack, %esp

/* Use 16-bit protected mode segment selectors */
mov $__KERN_DS16, %eax
mov $__RMODE_DS16, %eax
mov %eax, %ds
mov %eax, %es
mov %eax, %gs
mov %eax, %fs
mov %eax, %ss

/* Jump to real mode */
ljmp $__KERN_CS16, $.Lreal_mode
ljmp $__RMODE_CS16, $.Lreal_mode

.code16
.align 16
Expand Down
6 changes: 6 additions & 0 deletions arch/x86/segment.c
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Expand Up @@ -41,7 +41,10 @@ gdt_desc_t __data_init boot_gdt[NR_BOOT_GDT_ENTRIES] __aligned(16) = {
[GDT_NULL].desc = GDT_ENTRY(0x0, 0x0, 0x0),
[GDT_KERN_CS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL0, S, CODE, R, A), 0x0, 0xfffff),
[GDT_KERN_DS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL0, S, DATA, W, A), 0x0, 0xfffff),
#if defined(__x86_64__)
/* We only need this entry to transition to long mode. Hence, no use for any DS64. */
[GDT_KERN_CS64].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL0, S, CODE, R, A), 0x0, 0x00000),
#endif
/* clang-format on */
};

Expand All @@ -68,7 +71,10 @@ gdt_desc_t __data_rmode rmode_gdt[NR_RMODE_GDT_ENTRIES] __aligned(16) = {
[GDT_NULL].desc = GDT_ENTRY(0x0, 0x0, 0x0),
[GDT_KERN_CS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL0, S, CODE, R, A), 0x0, 0xfffff),
[GDT_KERN_DS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL0, S, DATA, W, A), 0x0, 0xfffff),
#if defined(__x86_64__)
/* We only need this entry to transition to long mode. Hence, no use for any DS64. */
[GDT_KERN_CS64].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL0, S, CODE, R, A), 0x0, 0x00000),
#endif
[GDT_RMODE_CS16].desc = GDT_ENTRY(DESC_FLAGS(P, DPL0, S, CODE, R, A), 0x0, 0xfffff),
[GDT_RMODE_DS16].desc = GDT_ENTRY(DESC_FLAGS(P, DPL0, S, DATA, W, A), 0x0, 0xfffff),
/* clang-format on */
Expand Down
11 changes: 7 additions & 4 deletions arch/x86/traps.c
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Expand Up @@ -90,13 +90,16 @@ static void init_gdt(percpu_t *percpu) {
percpu->gdt[GDT_NULL].desc = GDT_ENTRY(0x0, 0x0, 0x0);
percpu->gdt[GDT_KERN_CS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL0, S, CODE, R, A), 0x0, 0xfffff);
percpu->gdt[GDT_KERN_DS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL0, S, DATA, W, A), 0x0, 0xfffff);
#if defined(__x86_64__)
percpu->gdt[GDT_KERN_CS64].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL0, S, CODE, R, A), 0x0, 0x00000);
percpu->gdt[GDT_KERN_DS64].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL0, S, DATA, W, A), 0x0, 0x00000);

#endif
percpu->gdt[GDT_USER_CS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL3, S, CODE, R, A), 0x0, 0xfffff);
percpu->gdt[GDT_USER_DS32].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL3, S, DATA, W, A), 0x0, 0xfffff);
percpu->gdt[GDT_USER_CS64].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL3, S, CODE, R, A), 0x0, 0xfffff);
percpu->gdt[GDT_USER_DS64].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL3, S, DATA, W, A), 0x0, 0xfffff);
percpu->gdt[GDT_USER_DS].desc = GDT_ENTRY(DESC_FLAGS(GR, SZ, P, DPL3, S, DATA, W, A), 0x0, 0xfffff);
#if defined(__x86_64__)
percpu->gdt[GDT_USER_CS64].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL3, S, CODE, R, A), 0x0, 0xfffff);
percpu->gdt[GDT_USER_SYSEXITQ].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL3, S, DATA, W, A), 0x0, 0xfffff);
#endif

/* Assign identity mapping of the percpu, because GDT has only 32-bit base. */
percpu->gdt[GDT_PERCPU].desc = GDT_ENTRY(DESC_FLAGS(GR, L, P, DPL0, S, CODE, R, A), virt_to_paddr(percpu), PAGE_SIZE);
Expand Down
85 changes: 81 additions & 4 deletions common/usermode.c
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Expand Up @@ -23,9 +23,11 @@
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include <lib.h>
#include <mm/vmm.h>
#include <pagetable.h>
#include <percpu.h>
#include <processor.h>
#include <traps.h>
#include <usermode.h>

extern char exception_handlers[], end_exception_handlers[];
Expand Down Expand Up @@ -75,8 +77,10 @@ static void init_syscall(void) {
msr_star_t star;

star.eip = _u(_ul(&syscall_handler_entry));
star.kern_cs = __KERN_CS64;
star.user_cs = __USER_CS64;

/* see segment.h for details regarding these values */
star.kern_cs = __KERN_CS;
star.user_cs = __USER_CS32;

wrmsr(MSR_STAR, star.reg);
wrmsr(MSR_LSTAR, _ul(&syscall_handler_entry));
Expand All @@ -87,6 +91,12 @@ static void init_syscall(void) {
wrmsr(MSR_EFER, rdmsr(MSR_EFER) | EFER_SCE);
}

static void init_sysenter(percpu_t *percpu) {
wrmsr(MSR_SYSENTER_CS, _ul(__KERN_CS));
wrmsr(MSR_SYSENTER_ESP, _ul(percpu->tss.rsp0));
wrmsr(MSR_SYSENTER_EIP, _ul(&sysenter_handler_entry));
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}

void init_usermode(percpu_t *percpu) {
vmap_user_4k(&cr3, virt_to_mfn(&cr3), L1_PROT);
vmap_user_4k(&user_cr3, virt_to_mfn(&user_cr3), L1_PROT);
Expand All @@ -101,10 +111,30 @@ void init_usermode(percpu_t *percpu) {
vmap_user_4k(usermode_helpers, virt_to_mfn(usermode_helpers), L1_PROT);

init_syscall();
init_sysenter(percpu);
set_intr_gate(&percpu->idt[SYSCALL_INT], __KERN_CS, _ul(int80_handler_entry),
GATE_DPL3, GATE_PRESENT, 0);
}

static inline long __user_text _int80(long syscall_nr, long arg1, long arg2, long arg3,
long arg4, long arg5) {
register long return_code asm(STR(_ASM_AX));
register long _arg4 asm("r8") = arg4;
register long _arg5 asm("r9") = arg5;

/* clang-format off */
asm volatile(
"int $" STR(SYSCALL_INT) "\n"
: "=a"(return_code)
: "0"(syscall_nr), "S"(arg1), "d"(arg2), "D" (arg3), "r"(_arg4), "r"(_arg5)
);
/* clang-format on */

return return_code;
}

static inline long __user_text syscall(long syscall_nr, long arg1, long arg2, long arg3,
long arg4, long arg5) {
static inline long __user_text _syscall(long syscall_nr, long arg1, long arg2, long arg3,
long arg4, long arg5) {
register long return_code asm(STR(_ASM_AX));
register long _arg4 asm("r8") = arg4;
register long _arg5 asm("r9") = arg5;
Expand All @@ -121,6 +151,53 @@ static inline long __user_text syscall(long syscall_nr, long arg1, long arg2, lo
return return_code;
}

static inline long __user_text _sysenter(long syscall_nr, long arg1, long arg2, long arg3,
long arg4, long arg5) {
register long return_code asm(STR(_ASM_AX));
register long _arg4 asm("r8") = arg4;
register long _arg5 asm("r9") = arg5;
register long _arg3 asm("r10") = arg3;
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/* clang-format off */
asm volatile (
"mov %%" STR(_ASM_SP) ", %%" STR(_ASM_CX) "\n"
"lea 1f(%%" STR(_ASM_IP) "), %%" STR(_ASM_DX) "\n"
"sysenter\n"
"1: "
: "=a"(return_code)
: "0"(syscall_nr), "S"(arg1), "D" (arg2), "r" (_arg3), "r"(_arg4), "r"(_arg5)
: STR(_ASM_CX), STR(_ASM_DX)
);
/* clang-format on */

return return_code;
}

static __user_data syscall_mode_t sc_mode = SYSCALL_MODE_SYSCALL;

bool __user_text syscall_mode(syscall_mode_t mode) {
if (mode > SYSCALL_MODE_INT80) {
return false;
}
sc_mode = mode;

return true;
}

static long __user_text syscall(long syscall_nr, long arg1, long arg2, long arg3,
long arg4, long arg5) {
switch (sc_mode) {
case SYSCALL_MODE_SYSCALL:
return _syscall(syscall_nr, arg1, arg2, arg3, arg4, arg5);
case SYSCALL_MODE_SYSENTER:
return _sysenter(syscall_nr, arg1, arg2, arg3, arg4, arg5);
case SYSCALL_MODE_INT80:
return _int80(syscall_nr, arg1, arg2, arg3, arg4, arg5);
default:
UNREACHABLE();
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}
}

#define syscall0(nr) syscall((nr), 0, 0, 0, 0, 0)
#define syscall1(nr, a1) syscall((nr), (a1), 0, 0, 0, 0)
#define syscall2(nr, a1, a2) syscall((nr), (a1), (a2), 0, 0, 0)
Expand Down
8 changes: 8 additions & 0 deletions include/arch/x86/asm-macros.h
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Expand Up @@ -170,6 +170,14 @@
#endif
.endm

.macro SYSEXIT
#if defined(__x86_64__)
sysexitq
#else
sysexit
#endif
.endm

.macro SET_CR3 val
push %_ASM_AX
mov (\val), %_ASM_AX
Expand Down
13 changes: 13 additions & 0 deletions include/arch/x86/processor.h
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Expand Up @@ -124,6 +124,10 @@
#define EFER_FFXSR (_U64(1) << 14) /* Fast FXSAVE/FXRSTOR */
#define EFER_TCE (_U64(1) << 15) /* Translation Cache Extension */

#define MSR_SYSENTER_CS 0x174
#define MSR_SYSENTER_ESP 0x175
#define MSR_SYSENTER_EIP 0x176

#define MSR_STAR 0xc0000081
#define MSR_LSTAR 0xc0000082
#define MSR_CSTAR 0xc0000083
Expand Down Expand Up @@ -286,6 +290,15 @@ union msr_star {
};
typedef union msr_star msr_star_t;

typedef uint16_t msr_sysenter_cs_t;
#if defined(__x86_64__)
typedef uint64_t msr_sysenter_esp_t;
typedef uint64_t msr_sysenter_eip_t;
#else
typedef uint32_t msr_sysenter_esp_t;
typedef uint32_t msr_sysenter_eip_t;
#endif

static inline bool has_error_code(uint32_t vector) {
return !!((1U << vector) & X86_EX_HAS_ERROR_CODE);
}
Expand Down
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